This invention relates to thermo-magnetically operated switches which utilize the saturation flux density versus the temperature characteristic of a magnetic substance to control the switching temperature, and, in particular, to improvements in thermomagnetically operated switches having two different and predetermined lower and higher operating points on a temperature axis so that it may be maintained open below the lower operating point and above the higher operating points. This type will be referred to as an open-close-open type.
Mr. H. Satoh, one of the joint inventors of this invention, proposed thermo-magnetically operated switches having two different operating points, in U.S. Pat. No. 3,895,238 together with three other inventors Mr. Kato, Mr. Endo and Mr. Horiuchi, wherein two permanent magnets and two kinds of magnetic members having different Curie points are assembled on the outer surface of a reed switch.
In the arrangements of FIGS. 4A-8B of the aforesaid U.S. patent, two magnets are disposed in cascade alongside a reed switch with a high temperature sensitive ferromagnetic body interposed between the magnets, and a low temperature sensitive ferromagnetic body or bodies are overlapped on the two magnets. At a temperature below the lower operating point, magnetic flux from the magnets flows in a closed loop through the two ferromagnetic substances so that the reed switch is maintained open. At an elevated temperature above the lower operating point, magnetic flux from the magnets flows through both reeds to close the reed switch. At a further elevated temperature above the higher operating point, magnetic flux from respective magnets flows through respective reeds so that the reed switch is open.
However, in the arrangements as shown in FIGS. 4A-8B of the U.S. patent, a part of the magnetic flux is apt to leak to the reeds even if the temperature is below the lower operating point. As a result, the switch is erroneously closed. For this reason, exchange of magnetic-flux flow path from the reeds to the low temperature sensitive ferromagnetic body is not always effected at a predetermined temperature at a time of the temperature drop, and the restoring point varies.